A drive device which drives a motor or the like by converting commercial three-phase AC power to DC power is known. It is important for such a drive device to accurately detect occurrence of instantaneous power failure in order to safely operate equipment in the case where power failure of an AC power supply occurs. A power supply device for detecting instantaneous power failure of an AC power supply is known (for example, Patent literature 1: JP-A-2008-128897).
FIG. 1 illustrates a conventional motor drive device. A conventional motor drive device 100 has an AC power supply 101 and a DC power supply circuit 103 for converting three-phase AC voltages VR, VS and VT from an AC power supply 101 to DC voltages. The DC power supply circuit 103 has a converter 102 for converting the AC voltages received from the AC power supply 101 to DC voltages, an inverter 107 for converting the DC voltages to AC voltages, and a DC link smoothing capacitor 104 for smoothing the waveform of the DC voltage. The DC power which is output from the converter 102 is supplied to the inverter 107 via DC power supply lines 105 and 106, and a motor 113 is driven by AC power which is output from the inverter 107.
The motor drive device 100 has an instantaneous power failure detection circuit 108 for detecting an instantaneous power failure. The instantaneous power failure detection circuit 108 has a level converting circuit 109 and a control unit 110. The level converting circuit 109 converts all of the three-phase AC voltages VR, VS and VT to a level which can be measured by the control unit 110. The control unit 110 detects whether an instantaneous power failure occurs or not on the basis of AC voltage converted by the level converting circuit 109 and, in the case where an instantaneous power failure is detected, outputs an instantaneous power failure detection signal 111. With such a configuration, the conventional motor drive device 100 detects an instantaneous power failure. Patent literature 1 discloses a technique of detecting an instantaneous power failure of an AC power supply on the input side in a DC power supply circuit which performs AC/DC conversion. In Patent literature 1, however, protection of a power element in a converter from a damage and maintenance of continuous operation are not described.
When an instantaneous power failure occurs, DC link voltage VDC which is the voltage between the terminals of the DC link smoothing capacitor 104 decreases and the power is recovered from the power failure, current called inrush current flows in the DC link smoothing capacitor 104. In this case, the larger the decrease amount of the DC link voltage is and the larger the total capacity of the DC link smoothing capacitor 104 and a DC link smoothing capacitor 112 is, the larger the inrush current is.
With reference to FIGS. 2A to 2C and FIGS. 3A to 3C, the relation between the decrease amount of the DC link voltage and inrush current will be described. FIGS. 2A, 2B, and 2C illustrate changes with time of AC voltage, DC link voltage, and alternating current, respectively, in the case where the decrease amount of the DC link voltage is large. FIGS. 3A, 3B, and 3C illustrate changes with time of AC voltage, DC link voltage, and alternating current, respectively, in the case where the decrease amount of the DC link voltage is small. In the cases of FIGS. 2A to 2C and FIGS. 3A to 3C, it is assumed that the capacities of the DC link smoothing capacitors are the same respectively.
FIG. 2A illustrate changes with time of AC voltages VR, VS and VT. It is assumed that a power failure occurs at time t0 and the power supply recovers from the power failure at time t1. In the period from the time t0 to the time t1 [s] as an instantaneous power failure period, the AC voltages VR, VS and VT are 0[V].
FIG. 2B illustrate changes with time of the DC link voltage VDC [V]. It is understood that the DC link voltage VDC is substantially constant until time t0 and starts decreasing from the time t0 at which an instantaneous power failure occurs. After the power supply recovers at time t1, the DC link voltage VDC starts increasing and, after that, repeats increasing/decreasing (ringing) for a predetermined period, and is converged to a predetermined value.
FIG. 2C illustrate changes with time of alternating currents IR, IS and IT. In the period from the time t0 to the time t1 [s] as an instantaneous power failure period, the alternating currents IR, IS and IT are 0[A]. After the power supply recovers at time t1, inrush current flows. As an element in the converter unit, a power element having resistance to large current has to be used so that the device is not influenced even in the case where the inrush current occurs. In particular, in the IEC (International Electrotechnical Commission) 60204-1 standard, it is requested that equipment operates safely even in the case where instantaneous power failure in an AC power supply continues for 3 [ms] at the maximum. Therefore, it is desirable that the converter operates normally even in the case where the power supply recovers from an instantaneous power failure in an AC power supply after 3 [ms] since the start of the power failure.
Next, description will be made for the relation between the decrease amount of the DC link voltage and inrush current in the case where the decrease amount of the DC link voltage VDC is smaller than that in FIGS. 2A, 2B and 2C. FIG. 3A illustrates changes with time of AC voltages VR, VS and VT. It is assumed that a power failure occurs at time t0 and the power supply recovers from the power failure at time t1. In a manner similar to FIG. 2A, in the period from the time t0 to the time t1 [s] as an instantaneous power failure period, the AC voltages VR, VS and VT are 0[V].
FIG. 3B illustrate changes with time of the DC link voltage VDC [V]. The DC link voltage VDC is substantially constant until time t0, starts decreasing from the time t0 at which an instantaneous power failure occurs and, after that, is converged to a predetermined value.
FIG. 3C illustrate changes with time of alternating currents IR, IS and IT. In the period from the time t0 to the time t1 [s] as an instantaneous power failure period, the alternating currents IR, IS and IT are 0[A]. After the power supply recovers at time t1, inrush current flows. However, the intensity of the inrush current is smaller than that in the case of FIG. 2C. In such a manner, the intensity of the inrush current which occurs at the time of an instantaneous power failure largely fluctuates according to the decrease amount of the DC link voltage. The intensity I of the inrush current can be expressed as I=C×(dVDC/dt) using time rate of change dVDC/dt of the decrease amount of the DC link voltage and the capacity C of the DC link smoothing capacitor.
Subsequently, fluctuation in the DC link voltage VDC in the case where an instantaneous power failure occurs will be described. FIGS. 4A and 4B are diagrams illustrating the relation between the DC link voltage VDC and the level at which the power element may be destroyed, to determine whether the decrease amount of the DC link voltage VDC lies in a predetermined range or not with respect to both of the case where the decrease amount of the DC link voltage VDC is large and the case where the decrease amount of the DC link voltage VDC is small. The level at which the power element may be destroyed denotes a voltage level at which the power element in the converter unit may be destroyed by inrush current which is caused by decrease in the DC link voltage VDC. In the case where the DC link voltage VDC becomes equal to or lower than the level at which the power element may be destroyed, in order to prevent flow of the inrush current by which the power element may be destroyed, an alarm is generated. In such a manner, destruction of the power element due to the inrush current caused by decrease in the DC link voltage VDC can be suppressed.
FIG. 4A illustrate changes with time of the DC link voltage in the case where the decrease amount of the DC link voltage is large. FIG. 4B illustrate changes with time of the DC link voltage in the case where the decrease amount of the DC link voltage is small. In the case where the decrease amount of the DC link voltage VDC is large, as illustrated in FIG. 4A, when it is assumed that a power failure occurs in the period from the time t0 to the time t1, the DC link voltage VDC after the power failure becomes lower than a DC link voltage VDC0 before the instantaneous power failure. It is understood that the DC link voltage VDC falls below a level VB at which the power element may be destroyed by the inrush current after the time t0 and the power element may be destroyed.
In the case where the decrease amount of the DC link voltage VDC is small, as illustrated in FIG. 4B, when it is assumed that a power failure occurs in the period from the time t0 to the time t1, after the power failure, the DC link voltage VDC becomes lower than the DC link voltage VDC0 before the instantaneous power failure. It is understood that the DC link voltage VDC does not become below the level VB at which the power element may be destroyed by the inrush current, and there is no possibility that the power element is destroyed.
As described, in the case where the DC link voltage decreases at the time of an instantaneous power failure, when the power supply recovers, inrush current flows from the power supply toward the DC link. It is necessary to prevent a power element as a component of the converter unit from being influenced by the inrush current at the time of an instantaneous power failure. Generally, a power element for rectification having sufficient resistance is used for the converter unit so as not to be damaged by the inrush current. The inrush current becomes large when the capacity of a DC link smoothing capacitor is large and the DC link voltage decreases at the time of an instantaneous power failure. Consequently, in consideration of the worst conditions, an element having an excessive margin in normal operation has to be selected as a power element for rectification. In addition, in recent years, axes are becoming multiple, the capacity of DC link smoothing capacitors is increasing by parallel connection of units, and it is becoming difficult to select a power element having an excessive margin.
Patent Literature 1: JP-A-2008-128897
An object of the present invention is to provide a motor drive device capable of selecting a proper power element by making a protection level variable so that the power element is not destroyed by inrush current after recovery from an instantaneous power failure while satisfying a condition of operation continuation of a converter, which is equal to or longer than 3 [ms] of an instantaneous power failure as requested in IEC60204-1.